Recently, so-called build-up multilayer circuit boards have addressed from a demand for high densification of multilayer circuit boards. This build-up multilayer circuit board is produced by a method as described, for example, in JP-B-4-55555. That is, an interlaminar insulating resin agent composed of a photosensitive adhesive for electroless plating is applied onto a core substrate, dried, exposed to a light and developed to form an interlaminar insulating resin layer having openings for viaholes, and then the surface of the interlaminar insulating resin layer is roughened by treating with an oxidizing agent or the like, and a plating resist is formed on the roughened surface by subjecting a photosensitive resin layer to light exposure and development treatments, and thereafter a non-forming portion of the plating resist is subjected to an electroless plating to form a conductor circuit pattern inclusive of viaholes. Such steps are repeated plural times to obtain a multilayered build-up circuit board through an additive process.
In the build-up circuit board produced by such a method, the adhesive for electroless plating is obtained by dispersing soluble cured resin particles consisting of rough particles having an average particle size of 2-10 .mu.m and fine particles having an average particle size of not more than 2 .mu.m into a heat-resistant resin matrix hardly soluble through a curing treatment as disclosed in JP-A-63-158156 and JP-A-2-188992 (U.S. Pat. No. 5,055,321, U.S. Pat. No. 5,519,177) is used in the interlaminar insulating resin layer.
Furthermore, JP-A-61-276875 (U.S. Pat. No. 4,752,499, U.S. Pat. No. 5,021,472) discloses an adhesive for electroless plating obtained by dispersing soluble cured epoxy resin powder ground into an average particle size of 1.6 .mu.m in a hardly soluble heat-resistant resin matrix.
The interlaminar insulating resin layer formed on the substrate using the above adhesive is roughened on its surface by dissolving and removing the heat-resistant resin particles in the surface layer and is excellent in adhesion to a conductor circuit formed on the roughened surface through the plating resist.
However, the build-up circuit board retaining the plating resist as a permanent resist such as the circuit board produced by the full-additive process is bad in adhesion at the boundary between the permanent resist and the conductor circuit. Therefore, this build-up circuit board has a problem that when an IC chip is mounted on the board, cracks are generated in the interlaminar insulating resin layer starting from a boundary between the plating resist and the conductor circuit resulting from a difference of thermal expansion coefficient.
On the contrary, a method of removing a plating resist and subjecting at least a side face of a conductor circuit to a roughening treatment to improve adhesion to an interlaminar insulating resin layer formed on the conductor circuit has hitherto been proposed as a technique capable of reducing cracks created on the interlaminar insulating resin layer. As a method of producing a circuit board advantageously utilizing this method, mention may be made of the semi-additive process.
In the semi-additive process, the surface of the interlaminar insulating resin layer is first roughened and an electroless plated film is thinly formed over a full roughened surface and then a plating resist is formed on a non-conductor portion of the electroless plated film and further an electrolytic plated film is thickly formed on a no-resist forming portion and thereafter the plating resist and the electroless plated film located below the plating resist are removed to form a conductor circuit pattern.
However, the build-up circuit board produced through the semi-additive process using the above adhesive has a problem that the electroless plated film remains in a depression (anchor) of the roughened surface of the adhesive layer located under the resist lowering the insulation reliability between the lines.
The also, the build-up circuit board produced through the full-additive process using the above adhesive has a problem that the value of insulation resistance between the conductor circuits lowers under high temperature and high humidity conditions.
Moreover, the circuit boards produced through the full-additive process and semi-additive process have a problem that the interlaminar insulation is broken if the adhesive contains relatively large heat-resistant resin particles having an average particle size of not less than 2 .mu.m.
On the other hand, the above-mentioned adhesive for electroless plating, in case the printed circuit boards are industrially mass produced, require preservation from the beginning of manufacture up to the actual process of coating on the substrate.
For this reason, the adhesive for electroless plating has drawbacks including that a curing gradually proceeds or viscosity becomes high due to gelation during preservation.